Preparation of Soluble Salts

Soluble salts can be made by reacting acids with different substances. The goal is to make the salt, remove anything extra, and grow clean crystals.

Key idea: Neutralisation

A salt forms when an acid reacts and its hydrogen ions are removed. General equations:

$$\text{acid} + \text{alkali} \rightarrow \text{salt} + \text{water}$$

$$\text{acid} + \text{insoluble base} \rightarrow \text{salt} + \text{water}$$

$$\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen}$$

$$\text{acid} + \text{carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$$

Choosing the method

• Acid + alkali (both soluble): use titration (you cannot filter off an excess liquid).
• Acid + excess solid: if the other reactant is a metal, an insoluble base (e.g. metal oxide/hydroxide), or an insoluble carbonate, add it in excess, then filter.

Method A: Acid + Alkali (by titration)

• Measure a known volume of alkali into a conical flask. Add a few drops of indicator (e.g. phenolphthalein).
• Fill a burette with acid. Add acid slowly until the indicator shows neutralisation (end-point). Record the volume.
• Repeat without indicator using the same volumes to make a pure salt solution.
• Gently heat to concentrate, then cool for crystals. Filter, wash with a little cold distilled water, and dry between filter papers.

Method B: Acid + Excess Metal

• Warm the acid gently. Add small pieces of metal until bubbling (hydrogen) stops and some metal remains.
• Filter to remove unreacted metal. Evaporate, cool to crystallise, then filter, wash, and dry.

Method C: Acid + Excess Insoluble Base (oxide/hydroxide)

• Warm the acid. Add the base in small portions with stirring until some remains and no more dissolves.
• Filter off the excess solid, then crystallise as above.

Method D: Acid + Excess Insoluble Carbonate

• Add carbonate to warm acid. Effervescence shows CO₂ gas.
• When fizzing stops and some solid remains, filter. Then evaporate, cool, filter crystals, wash, dry.

Common misconceptions

• Do not leave indicator in the final crystals (repeat titration without indicator).
• Washing crystals uses a very small amount of cold distilled water; too much will dissolve them.
• Overheating to dryness can destroy water of crystallisation; gently evaporate then cool to crystallise.
• You cannot make all salts with metals (some metals are too reactive or unsafe with acids).